White Underbase Screen Printing: Opacity, Flash and Choke
White underbase screen printing for dark garments: when you need it, flash and opacity, choke basics and soft-hand tradeoffs.
Shop playbook for white underbase, flash, choke and opacity on dark shirts.
Short answer: in white underbase screen printing, the underbase is a white or tinted ink layer printed and flashed under colors on dark garments so top colors stay true; opacity, a light flash to gel (not full cure), and a slight choke keep the print bright without a muddy or overly thick hand.
Rule: use an underbase when garment color would mute or shift the design, flash only until the surface is dry to the touch, choke the underbase slightly inside the top colors, and keep the deposit as thin as opacity allows. Check manufacturer or supplier specs for flash distance, mesh and cure.
Part of the screen printing guides in the DAX Learning Hub. Start with screen printing for beginners, then see plastisol vs water based ink, the screen printing mesh count chart, flash vs conveyor dryer, and how to cure plastisol ink.
What White Underbase Screen Printing Is
An underbase (also called an underlay) is a layer of ink printed first so later colors sit on a sealed, lighter surface instead of raw dark fabric. White is common because it restores brightness. Pros also use gray, tinted or clear underbases when the job needs less contrast or a softer tonal look. The underbase is not the same thing as a highlight white that appears as white in the finished design.
When You Need an Underbase (and When You Do Not)
You usually need an underbase when light or translucent colors print on dark or medium garments and would otherwise look dull or shift. You may skip or reduce it when high-opacity inks and design intent already deliver the look, when a vintage muted look is the goal, or when only part of the art sits on dark areas. Simulated process and multi-tone work often use a tonal underbase rather than a solid sheet of white. For process choices, see simulated process vs spot color. On polyester and blends, underbase and low-bleed chemistry also interact with dye migration risk; see dye migration on polyester.
| Situation | Underbase lean | Why |
|---|---|---|
| Bright spot colors on black or navy cotton | Usually yes | Garment color would mute top inks |
| High-opacity athletic block on dark | Maybe thinner or partial | Opacity and hand trade off |
| Simulated process on dark | Often tonal, not solid white | Solid white can crush detail |
| Vintage washed look | Often no or light | Muted hand is intentional |
| White in the art on dark | Underbase plus separate highlight white | Using underbase as highlight lays too much ink |
How to Print a White Underbase on a Dark Garment
- Separate the underbase. Build a dedicated underbase plate. If white appears in the design, plan a separate highlight white.
- Choose mesh for opacity and detail. Trade guides commonly start solid white underbases in a mid-open band (often roughly 110 to 160 threads per inch), with finer mesh for detail or simulated process. Confirm against your ink and the mesh count chart.
- Expose and set the screen. Coat and expose for a clean stencil. Set off-contact low enough to clear without dragging.
- Print the white. Aim for enough opacity to seal fibers without a bulletproof deposit. Stir thick white well; avoid dumping reducer into white just to make it easier if that kills opacity.
- Flash to gel, not full cure. Flash until the surface is dry to the touch and not tacky. Over-flashing can cure the underbase and hurt overprint adhesion.
- Check opacity. Look for fiber show-through and uneven coverage before you add colors.
- Print top colors with choke as needed. Keep the underbase slightly inside the overprints so white does not peek at edges.
- Cure and wash-test. Final-cure on the conveyor per ink maker guidance, then wash-test a sample before the full run.
Flash, Opacity and Soft-Hand Tradeoffs
Flash only long and hot enough to gel the surface. Trade education repeatedly warns that cooking the underbase (surface temperatures high enough to fully fuse it) causes poor intercoat adhesion and wash failure of top colors. Exact flash time, height and surface temperature depend on the unit, ink and garment thickness, so measure with a non-contact thermometer and document what works in your shop. Thicker deposits need more flash energy and can feel stiff; thinner deposits feel softer but may show garment color. Soft-hand goals usually mean the finest mesh and lightest deposit that still meet opacity.
Choke, Trap and Registration Basics
Choke means making the underbase slightly smaller than the colors that print over it so white does not show as a halo. Trade how-tos often describe about one to two points of choke or a slight spread of the overprint, adjusted for your press variation. Trap and butt-register techniques for colors that meet on a flashed underbase also matter: a solid flashed underbase behaves like a non-absorbent plastic sheet, so wet-on-wet overlays can mottle unless you design gaps or use a tonal underbase. Keep screens locked and check registration after flash.
Underbase vs DTF or DTG White Layers
A screen underbase is a flashed ink film on the garment that other screen colors print over. DTF builds a white layer on film with powder adhesive, then presses the transfer. DTG typically pretreats the garment, then prints white and colors digitally. All three create a light base for color on darks, but chemistry, hand, setup cost and failure modes differ. Do not treat DTF or DTG white settings as screen flash recipes.
Frequently Asked Questions About White Underbase Screen Printing (FAQ)
What is a white underbase in screen printing?
It is a white or light ink layer printed first on a darker garment, then gelled with a flash, so later colors stay bright and opaque instead of sinking into the fabric color.
Do you always need an underbase on dark shirts?
No. High-opacity inks, partial coverage, vintage looks and some specialty designs can skip or reduce it. Evaluate garment color, ink opacity and hand requirements per job.
What is a choke underbase?
A choke underbase is sized slightly smaller than the top colors so the white stays hidden under the overprints and small registration shifts do not show a white edge.
How many times should you flash an underbase?
Usually once to gel, then print colors. Extra print-flash-print cycles on white thicken the hand and can complicate cure. Follow your ink maker and dryer setup rather than a fixed second count.
Can water based ink use an underbase?
Some water-based workflows use underbases or discharge strategies, but flash behavior and chemistry differ from plastisol. Treat water-based underbase as its own system and follow the ink maker tech sheet. Compare systems in plastisol vs water based ink.
Flash temperatures, mesh counts, choke amounts and cure times vary by ink, flash unit, mesh and garment. Always check the supplier or manufacturer tech sheet for your products. This page uses generic terms only and does not name ink brands.
More screen printing guides live in the DAX Learning Hub. Compare equipment and supplies in person on the DAX show schedule.